Noise generally is classified as continuous (exhibiting only small changes in level over time), intermittent (interrupted by occasional increases in level), impulsive (containing components with sharp rises and rapid decays), or complex (a combination of the above), and the frequency range and level can vary with the type and source of the noise.
Noise-induced hearing loss (NIHL) is hearing loss caused by loud sounds. NIHL can be caused by a single exposure to an intense “impulse” sound, such as an explosion, or by repeated or continuous exposure to loud sounds over an extended period of time, such as noise generated in a woodworking shop. NIHL is not understood completely, but current models of NIHL suggest that sounds at levels above about 85 dB are likely to damage sensitive structures in the inner ear, leading to hearing loss. Current models of NIHL also suggest that extremely loud impulsive sounds (sounds with rise times shorter than about one second and peak amplitudes over about 85 dB) cause damage more quickly than softer sounds with longer rise times. Loud, impulsive sounds may also cause tinnitus, a condition in which the afflicted person perceives ringing in the ears even under silent conditions.
NIHL affects up to 15% of Americans between the ages of 20 and 69, or about 26 million people total. More than 30,000 cases of noise-induced hearing injuries were reported among active-duty soldiers, sailors, airmen, and Marines in 2010. The number of new tinnitus incidents per year increased 42% from 2007-2010 among service members. In 2009 alone, the government made more than 100,000 new service-connected disability awards for tinnitus and hearing loss. About 10% of veterans' disability payments made for tinnitus and hearing loss; in 2013, hearing loss and tinnitus disability payments totaled about $850,000,000 per year.
Dosimetry involves measuring sound pressure levels (SPLs) in a noise environment with the goal of estimating the total dosage to which an individual is exposed over a period of time. Often the dose is estimated in terms of A-weighted energy in conjunction with the equal-energy hypothesis (EEH), which assumes that accumulated noise energy is sufficient to determine risk of NIHL and that the underlying temporal characteristics of the noise are irrelevant. Under the EEH, two exposures are equivalent if the respective average noise levels and durations comply with a specified exchange rate. For example, a 3-dB exchange rate often is employed such that a halving or doubling of the exposure time is accommodated with a +3 or −3 dB adjustment, respectively, to the allowable noise level.
In an effort to conserve hearing in industrial and military settings, guidelines on the maximum allowable daily noise exposure are recommended by regulating agencies such as National Institute for Occupational Safety and Health (NIOSH) and military branches under the U.S. Department of Defense (DoD) Hearing Conservation Program. This allowable daily noise dosage is expressed as a percent relative to the recommended limit, i.e., 100% dose represents maximum allowable noise exposure for an individual. For exposure in a continuous noise environment, the current military standard design criteria MIL-STD-1474E (2015) sets a limit of 85 dBA for a duration of 8 hours, where the exposure duration and level may be traded off to satisfy an equal-energy criterion using a 3 dB exchange rate.